Hi Louis (Louis Grau), in <199805011811_MC2-3BAF-DD10@compuserve.com> on May 1 you wrote:

> But DTMF
> decoding looks a lot more complicated (A/D converter + DFT?). A Fourrier
> transform must obviously
> eat up a lot of PIC ressources (execution time and code memory). Maybe
> someone has found a trick
> for detecting DTMF tones in a simpler way. Who knows?

There ARE ways to detect DTMF in a PIC, but they consume lots (almost all)
of CPU time and still do not meet the DTMF specs.  They distinguish the
digits, but can't tell DTMF from noise or voice.  Also, developing such an
(poor) algorithm that fits into a PIC is a job of several manweeks.

Better take one of the DTMF encoder/decoder chips, such as MITEL 88xx
series.  You can _reliably_ detect and send quality DTMF tones in minutes!

> 2 - The phone is temporarily disconnected from the phone line (with a
> relay). But here, how can the dial
> tone, busy tone, operator voice (etc..) be heard by the person (or fax,
> modem) who makes the call.

I think MITEL has hybrid circuits that "simulate" a phone line, and they
definately have circuits that convert the outgoing line into Audio IN and
Audio OUT (MH88422 and others).  Probably one of the most expensive
solutions, but easy to develop with.  When you use ready made hybrid
modules you might even get FCC approval without much hassle (check with the
manufacturer).